Riyad-us-Saliheen 762
Abu Sa'id Al-Khudri (May Allah be pleased with him) reported: Messenger of Allah (PBUH) forbade turning the water-skin upside down and drinking directly from its mouth.
[Al-Bukhari and Muslim]
Messenger of Allah (PBUH) has forbidden us to drink water from the mouth of a water-skin or something similar to it because some harmful insect in the water-skin may flow into the stomach. So, instead of drinking direct from the mouth of the water-skin, it is better to pour it into a drinking vessel in order to shun any possible harm.